Recently, an improvement in fuel efficiency is expected to reduce the amount of carbon dioxide emission, which is considered as one cause of global warming, in the automotive industry. To reduce the amount of carbon dioxide emission, measures such as improving engine efficiency and transmission efficiency and reducing a weight of a vehicle body are expected in addition to a fundamental measure caused by adoption of alternative fuel. Conversely, development of a vehicle body having excellent collision safety satisfying requirements for collision safety regulations that have been made stricter is simultaneously expected.
A vehicle body satisfying requirements for collision safety regulations can be formed with a low-strength steel sheet, for instance, by arranging many reinforced components and increasing a sheet thickness of a blank for a component. However, since a weight of the vehicle body is increased by arranging the reinforced components and increasing the sheet thickness of the blank for the component, a request that the weight of the vehicle body be reduced is not satisfied. To satisfy conflicting requests that the weight of the vehicle body be reduced and collision safety performance be improved, a high-strength steel sheet is adopted as a steel sheet for the vehicle body of the vehicle. A steel sheet having a tensile strength of about 440 MPa has been adopted as the steel sheet for the vehicle body of the vehicle. However, in recent years, a 590 MPa-class steel sheet, and furthermore a 980 MPa-class steel sheet, has been adopted as the steel sheet for the vehicle body of the vehicle. The high-strength steel sheet is adopted as the steel sheet for the vehicle body of the vehicle so that the requests that the weight of the vehicle body be reduced and the collision safety performance be improved can be satisfied. However, as strength of the steel sheet has been improved, there is a possibility of reducing formability. Since a high-strength steel sheet has been adopted as the steel sheet for the vehicle body of the vehicle, an improvement in formability, and particularly an improvement in stretch flange formability, is expected. In general, stretch flange formability is evaluated with a limit hole expansion rate λ in a conical punch hole expansion test.
Since a stretch flange fracture is considered to occur when elongation strain of a flange end in a circumferential direction exceeds a limit value, steel sheets having an improved stretch flange formability, namely various steel sheets having a high hole expansion rate, are known.
A steel sheet in which stretch flange formability is improved by controlling a microstructure, such as ferrite or bainite, is described in Patent Document 1.
An aluminum alloy sheet having excellent stretch flange formability regulating uniform elongation in a specific direction in plastic anisotropy and a tensile test is described in Patent Document 2.
When a material is isotropic, the material is deformed in a hole expansion test while axial symmetry thereof is maintained. Therefore, in the hole expansion test of the isotropic material, an increase in elongation strain of a circumferential end of the isotropic material is uniform, and a local fracture limit major strain when a fracture occurs becomes a value corresponding to the limit hole expansion rate λ. However, a portion having a possibility of a stretch flange fracture occurring at a steel sheet which is actually used as the steel sheet for the vehicle body of the vehicle or the like is a portion at which an end of the steel sheet extends in axial symmetry, as in the hole expansion test, in addition to a portion at which the strain is distributed in a circumferential direction of the end of the steel sheet. For example, a stretch flange fracture occurs when a strain distribution occurs in the circumferential direction of the end of the steel sheet during the forming and when the localized strain exceeds a limit value of ductility of the material. The following two measures exist to prevent the occurrence of the stretch flange fracture attributed to the strain distribution in the circumferential direction of the end of the steel sheet.
(1) Improving fracture limit major strain by adopting a steel sheet having a good hole expansion rate.
(2) Suppressing localization of the strain by improving a die shape, forming conditions, and a method of construction to regularize the strain.
To regularize the stain, various forming methods of relieving strain concentration on a portion having a high possibility of causing a stretch flange fracture by optimizing a press process and adjusting a shape of the end edge portion of the component are known (for example, see Patent Documents 3 to 5).
A method of projecting the portion having a high possibility of causing a stretch flange fracture, performing pre-deformation to reduce an amount of change of a local length at a flange end thereof, and controlling a deformation history to suppress generation of the local strain when the strain leading to a press bottom dead center is increased is described in Patent Document 3.
A method of suppressing the local concentration of the strain by performing deformation history control on the strain distribution of the portion having a high possibility of causing an elongation strain fracture in a plurality of processes is described in Patent Document 4.
The methods described in Patent Documents 3 and 4 have a problem in that a die design and a process design are complicated and that an adoptable component shape is limited, and are not easily put into practical use.
A method of increasing a curvature radius of an end edge portion of a component of the portion having a high possibility of causing a stretch flange fracture, suppressing the strain concentration along a flange end in the circumferential direction, and preventing the generation of the stretch flange fracture is described in Patent Document 5. In the method described in Patent Document 5, a ratio between a radius R1, which has an offset with the same length as a flange directed to an end of the component from a die corner radius, and an end corner radius R2 is set to R2/R1≥2 so that the strain concentration at the end is suppressed. However, the portion at which the stretch flange fracture occurs when the steel sheet is actually used is included in an end edge portion having a shape in which a curve whose curvature is changed and a straight line are continuously formed. For this reason, an adjusting region of the end edge portion suppressing the strain concentration of the flange end, and versatile design guides of the end edge portion such as a curvature and a radius of the end edge portion are not easily set. The adjustment of the end edge portion suppressing the strain concentration of the flange end needs to repeatedly produce a prototype a plurality of times and has a possibility that a load of a designer is increased.
FIG. 1A is a view illustrating a first example of flange-up forming, and FIG. 1B is a view illustrating a second example of flange-up forming. In the example illustrated in each of FIGS. 1A and 1B, a shape of a component on which a hole expansion test is performed by a cylindrical punch is divided, and a forming test simulating the flange-up forming is performed. A die used in the forming test has a die shoulder R of 5 mm and a diameter of 106 mm, and the cylindrical punch has a shoulder R of 10 mm and a diameter of 100 mm. A blank used in the forming test is a steel sheet having a tensile strength of about 440 MPa and a sheet thickness of 1.4 mm. The blank is sheared to 180 mm square, and is then cut into quarters. Afterwards, a corner of the blank is machined with a punch having a radius of 30 mm in the example illustrated in FIG. 1A, or a radius of 60 mm in the example illustrated in FIG. 1B.
In the example illustrated in FIG. 1A, a stretch flange fracture occurs at an end of the component. In the example illustrated in FIG. 1A, a strain of 0.36 in a circumferential direction is introduced into a fracture part so that a stretch flange fracture occurs. In the example illustrated in FIG. 1B, since a die corner radius has an end corner radius that is two times an offset radius, strain concentration of the end in a circumferential direction is reduced so that no crack occurs at a flange-up end. However, in the example illustrated in FIG. 1B, strain exceeding 0.3 is introduced to an end around a vertical wall passing the die shoulder and a crack occurs at the end around the vertical wall.